A simple technique is demonstrated based on UV–vis spectroscopy that permits the onset of fullerene crystallization to be monitored in polymer:fullerene bulk‐heterojunction blends. The growth of fullerene crystals only occurs during thermal annealing above the glass transition temperature (Tg) of the blend. Light scattering due to the presence of micrometer‐sized fullerene crystals reduces the apparent transmittance, which can serve as a clear indicator for the upper limit of the Tg. This principle is used to deduce this critical parameter for two high‐performance photovoltaic blends that are based on a thiophene–quinoxaline and a thiophene–isoindigo copolymer, respectively. High Tgs of 115 and 145 °C promise good thermal stability.